In most modern fuel cell systems, a compressor provides compressed air to the fuel cell stack. Having sufficient air for the fuel cell reaction is extremely important and is characterized as “Cathode Stoichiometry” wherein a higher value (e.g. 5) is typically needed at low current densities and a lower value (e.g. 1.8) is typical at high current densities. In such systems it is necessary to have a means for sensing the air mass flow rate leaving the compressor and entering the fuel cell stack, such as an air mass flow sensor.
A control system will typically take this flow information and change the speed of the compressor along with the position of a back pressure valve to achieve a desired air mass flow and gas pressure entering the fuel cell stack. The desired air mass flow and gas pressure are generally calculated using known factors such as the fuel cell stack current, number of cells in the fuel cell stack, and the desired cathode stoichiometry at that stack current.
In such fuel cell systems the control system typically allows an external circuit to draw current out of the fuel cell system immediately upon detection of the desired air mass flow rate by the air mass flow sensor. The volume and distance between the location where the air mass flow sensor is taking the measurement and the location where the air is required at the reaction site of the fuel cell stack are not taken into account. Therefore, the current is drawn out of the fuel cell stack before the desired air mass flow is actually present at the reaction site. The lack of air at the reaction site can cause the cathode stoichiometry at the reaction site to drop enormously, and lead to significant voltage drops in cells that are sensitive to low cathode stoichiometries. The lowered cell voltages can at least cause the power management circuit to limit power output and could reverse (i.e. negative voltage) causing massive degradation. The lack of air is particularly harmful on current draw up-transients. The prior art systems do not take into account the distance and volume between where the air mass flow meter is taking the measurement and where the air-H2 reaction actually takes place.
It would be desirable to develop a method of managing fuel cell power increases which would account for the volume and distance between the air mass flow sensor and the reaction site insuring the required air mass flow rate had reached the reaction site before the current is drawn from the fuel cell stack.